<!doctype html>
<!-- Divs should have smooth, continuous borders -->
<style>
div {
    display: inline-block;
    border: 20px #00F double;
    width: 100px;
    height: 100px;
    padding: 0px;
    margin: 10px;
}

div.tl {
    border-top-left-radius: 500px;
}

div.tr {
    border-top-right-radius: 500px;
}

div.bl {
    border-bottom-left-radius: 500px;
}

div.br {
    border-bottom-right-radius: 500px;
}

</style>
<div class="tl"></div>
<div class="tr"></div>
<div class="bl"></div>
<br>
<div class="br"></div>
<div class="tl br"></div>
<div class="tr bl"></div>

